It has a very short range, 10 m, acting over the nuclear distance scale. -15 The strong nuclear force is not felt by leptons, eg. electrons, and it is mediated by gauge bosons called gluons that pass between quarks. The photoelectric effect is the emission of electrons from a metal surface when light shines on it. The discovery of the photoelectric effect could not be explained by the electromagnetic theory of light. Albert Einstein developed the quantum theory of light in 1905. The Photoelectron Effect 6.63x10^-36 frequence If the photon energy is greater than the work function, a photoelectron will be ejected Light exhibits either wave characteristics or particle (photon) characteristics, but never both at the same time. The wave theory of light and the quantum theory of light are both needed to explain the nature of light and therefore complement each other. What is light? Wilhelm Roentgen (1845-1923) Wilhelm Roentgen accidentally discovered x-rays in 1895. In 1912, Max von Laue showed that x-rays are extremely high frequency em waves. X-rays are produced by high energy electrons that are stopped suddenly; the electron KE is transformed into photon energy. X-rays Louis de Broglie(1892-1987) In 1924, the French physicist Louis de Broglie proposed that moving objects behave like waves; these are called matter waves. The de Broglie wavelength of a particle of mass m and speed v is l = h/mv. De Broglie Waves When an electron "jumps" from one orbit (energy level) to another, the difference in energy between the two orbits is hf, where h is Planck’s constant and f is the frequency of the emitted or absorbed light. Electron Waves and Orbits Photons of light hitting a surface will transfer energy to the electrons Photon energy is related to the frequency of the light by the familiar E=hf. If the photon energy is greater than the work function, a photoelectron will be ejected.

This means that if hf => workfunction, photoelectrons will be released, so the threshold frequency occurs when hf = workfunction. The weak nuclear force acts on ALL particles, but over a short range (10x10 m, 10 times greater than gravitational force).

It is responsible for radioactive decay, and has 3 gauge bosons: W , W and Z.The W has a positive charge and the W has a negative charge. -18 33 + - + - The W gauge bosons have a short range, less than 0.001fm. The Electromagnetic Force acts between all charged particles (ions), therefore it is responsible for everything that happens to us (ie. friction, buoyancy, contact forces). The force is carried between charged particles by the photon (y). When 2 charged particles exert a force on each other, a virtual photon is exchanged between them. Electric wave Magnetic wave Gravity was discovered by Isaac Newton in the 1800's. The force acts on all objects and has an infinite range. It has negligible influence on atomic scale because it is the weakest of all fundamental forces.

Its gauge boson has not been discovered, but it is believed to exist with zero rest mass and zero charge and has been named the 'Graviton'. Baryons Baryons have either 3 quarks or 3 antiquarks U U d Protons U U d d Neutrons FOR EXAMPLE: Albert Einstein (1879-1955) Einstein expanded Planck's hypothesis by proposing that light could travel through space as quanta of energy called photons. Einstein's equation for the photoelectric effect is hf = KE + w. Although photons have no mass and travel with the speed of light, they have most of the other properties of particles. The higher the frequency (or shorter the wavelength) the higher the energy. Photons Mesons Mesons have one quark and one antiquark S U S Kaons U d Pions FOR EXAMPLE: U d Negative Positive Neutral d d Pions have no strangness d U Negative Positive Neutral S Kaons have strangness Baryons all eventually decay into protons Only contain 'Up' and 'Down' quarks d U Mesons do not have protons in their decay products Mesons can contain 'Strange' quarks in them as well as 'Up' and 'Down' quarks S Leptons Leptons are fundamental particles,meaning they have no quarks Muons Heaviest lepton Electrons Next mass down Neutrino Practically mass-less Rest energy/ MeV: 938.257